Atomic Absorption Spectroscopy (1 Viewer)

[Aerlinn]

I know that in both qualitative and quantitave analysis are involved in AAS, but where's the qualitative analysis part in it? I'm not quite sure I can pick it out.

With calibration curves in AAS, how do these curves correct for atoms excited by the flame heat (not the light source)? Eg. If you get an amount of these atoms, will you get a lower than 'expected' absorbence reading or something like that? But if that's the case, how do you know this 'lower than expected' reading is unusual/ unexpected if it's what you're trying to find in the first place? In other words, how does the calibration curve help in this?

The light source for AAS is a cathode lamp made from the metal being analysed. What I want to know is why the metal being analysed?

In help in sorting any of this out would be awesome ^_^

 

[xiao1985]

with the absorption pattern, you can tell which metals are present... that's qualitative....

as for the heat exciting the atom, i guess you can only argue in a handwaving way: that it only excite the minimum amount of atoms, and don't really influence the result...

but really, once the sample is atomised, even if a fraction is excited, they'd just relax straight way... that happens in the order of pico seconds to microseconds at most... which means even they are excited, it shouldn't influence the reading too much...

i dont' get this question you are asking... are you asking the fact that it won't be as accurate?! well science is never all accurate... if you think that's horrifying, try to do some pt... they try to use y = mx + b to explain everything in universe (no offence to all the p/t chemist out there)
moral of the story: there will be errors... just it is not yet (probably won't ever be) a problem.

last question: why the metal is being analysed:
scenario: average human can eat 80 ug of lead without dying, so govement say that "a can of tuna cannot contain mroe than 8ug of lead"
company A makes tuna for everyone...
govement want to find out if company a is complying with the regulation...
now what does the scientists do?! they use AAS... that's why metal are being analysed

 

[Aerlinn]

Aha, thanx. First up, yea, I was in a sense saying that the results will be less accurate, but I read that the calibration curves correct for that error... and so I wanted to know how that was done (to word it simpler) I confused myself trying to articulate that too, dun worry.

With my last question, I think you misread it. 'The light source for AAS is a cathode lamp made from the metal being analysed. What I want to know is why the metal being analysed?' As in, to re-word the question, why do they use the metal being analysed as the material for the lamp?
As for why the metal's being analysed, which you answered, nice example ^_^

 

[xiao1985]

why do we use metal in the lamp you ask?!

well it's easier... since for eg, say Pb absorbes (and hence emits) at 500nm and 700 nm, then all we need is to use a Pb lamp, which emit light at 500nm and 700 nm... because we will ONLY be observing this two (if not just one of the two) wavelengths... other waves will be redundant... (for eg, if we are testing Pb, we wont' be looking at wavelength of 680nm would we now?!)

 

[Nakashima]

why do we use metal in the lamp you ask?!

well it's easier... since for eg, say Pb absorbes (and hence emits) at 500nm and 700 nm, then all we need is to use a Pb lamp, which emit light at 500nm and 700 nm... because we will ONLY be observing this two (if not just one of the two) wavelengths... other waves will be redundant... (for eg, if we are testing Pb, we wont' be looking at wavelength of 680nm would we now?!)

I'm impressed that someone who graduated in 03 can still remember all that detail.

 

[Aerlinn]

Well, he's smart, I can tell ya that
--
Hmm, that makes sense. So each metal not only absorbs light at particular wavelengths, they also emit at those same wavelengths. Question. Can two metals ever absorb/emit the same wavelength of light?

 

[xiao1985]

I'm impressed that someone who graduated in 03 can still remember all that detail.

thank you thank you =)

@ aerlinn: theoretically, no two atoms can absorbe/emit at a same wavelength...

 

[Aerlinn]

No two atoms as in from two different elements or the same...?

What'd you mean by 'theoretically'? >.<

 

[xiao1985]

if Na absorbs at 300nm for eg, no other element will absorb at 300 nm... that's what it mean...

however, that doesn't guarantee no other elements will absorb at 301 nm and so on...

 

[Aerlinn]

Ah, so each element is VERY specific for the wavelengths it absorbs.

K, so back to the other one btw, do you know how a calibration curve corrects for the error that small amounts of the atoms are excited by the flame (rather than the light source)? Something to do with little shadows appearing on the curve in the printout, but since I've never actually done something like spectroscopy before, I dont know about that. Can you enlighten me as to how it corrects the errors...

 

[xiao1985]

last time i did AAS, we actually had salts and made solutions of them...

if the samples are to be atomised, then i suppose AAS machine will allow a lapse of time before the reading of absorptions are taken... since emiting of the light occurs very rapidly (in the order of nano seconds even pico seconds)

i'm not entirely sure what the shadow you are referring to... i must apologise as i do not specialise in AAS at all...

 

[Aerlinn]

That's alright. I read that you get a printout after doing AAS, which shows you the results. Maybe the diagram/curve lets you work out the concentration of the metal sample you are analysing. And apparently there are little shadows, like on tv screens, on the curve, if you happen to have errors during the process, eg. atoms excited by the flame when they're not supposed to.
Hmm... I don't think I follow you when you talking about allowing a 'lapse of time before the readings of the absorptions are taken', how does that relate to it?

 

[xiao1985]

flame excite metal atoms accidentally during atomisation,

then the atom give off light

AFTER THIS

we do AAS... which means there will not be any error given by the acciental excitation from the flame (since they have already emitted themselves...

again this is my theory... i'm no expert in AAS... so i'm just using my experience in explaining the matter...

 

[Aerlinn]

which means there will not be any error given by the acciental excitation from the flame (since they have already emitted themselves...

So, basically you say... there will appear to be no error though there might have been...

 

[xiao1985]

again, this is my speculation....

i am not an expert in AAS machine...

in fact, i really kinda detest any machineries... (with the exception of rot vap)